The present invention relates to a waveform control device for an electrical discharge machining apparatus.
A waveform control device for an electrical discharge machining apparatus supplies a pulsating current waveform, controlled to have a desired peak current value I.sub.P, between an electrode and a workpiece to establish an electrical discharge in accordance with a set ON-OFF time so that the workpiece may be machined in a desired form. It is desirable that the pulsating current waveform closely approximate a square wave pulse so that the machining operation can be accurate, thereby requiring fast rise times and fast discharge times.
FIG. 24 is a circuit diagram showing the waveform control device of the prior art, as exemplified by Japanese Kokai No. 49-43297 or Japanese Utility Model Publication No. 57-33950. In FIG. 24, an electrode 1 is positioned adjacent to a workpiece 2 to be machined. The two elements are connected in a circuit with a DC power source B.sub.1 having a voltage E.sub.1 (V). Also connected in the circuit is an auxiliary power source B.sub.2, having a voltage E.sub.2 (V), and a choke induction coil L.sub.1 constituting a magnetic energy reservoir. A switch S.sub.1 is connected in series between the auxiliary source B.sub.2 and the coil L.sub.1 for controlling the electric current flowing through the coil L.sub.1 to a desired value. Another switch S.sub.2, which is ON-OFF controlled by a pulse generator (not-shown) is connected in series between power source B.sub.1 and the workpiece 2. Diodes D.sub.1 and D.sub.2 are placed in the circuit to direct the current flow.
The ON-OFF timing of the switches S.sub.1 and S.sub.2 and the waveform of the electric current i.sub.L1 flowing through the coil L.sub.1 of the aforementioned circuit are illustrated in FIGS. 25 and 26. FIG. 25 shows the operation when the machining apparatus is performing a finishing step on a workpiece whereas FIG. 26 shows the operation when a rough machining step is performed. In each of these figures, the switches S.sub.1 and S.sub.2 assume states 1 and 0, which are conductive at 1 and non-conductive at 0.
With this circuit structure, the coil L.sub.1 is connected in series between the electrode 1 and switch S.sub.1 so that the coil acts as an energy reservoir. Accordingly, when the coil L.sub.1 is changed from its saturated to its unsaturated states or vice versa by operation of switches S.sub.1 and S.sub.2, the discharge current from the coil is abruptly changed. For example, if the switch S.sub.1 is ON, the current through the coil L.sub.1 substantially increases so long as the switch S.sub.2 is ON. If the switch S.sub.2 is OFF, while switch S.sub.1 is ON, the current of the coil L.sub.1 is maintained at a substantially constant level because a closed circuit through diode D.sub.2 is formed. If the switch S.sub.1 is turned OFF, any stored energy in the coil is released and the current of the coil L.sub.1 will decrease regardless of whether the switch S.sub.2 might be ON or OFF. When both switches S.sub.1 and S.sub.2 are OFF, the current flows in a circuit comprising power supply B.sub.1, diode D.sub.1, coil L.sub.1 and diode D.sub.2 and settles at a steady state level. When a discharge occurs across the gap between electrode 1 and workpiece 2, a current will flow and the voltage across the elements will drop. As a result, a waveform control circuit using a coil as a discharge current limiting element has advantages over a waveform control circuit using a resistor as the current limiting element since there is no resistor to consume energy.
In the circuit constructed in accordance with FIG. 24, however, there is a wiring inductance L.sub.3 that exists in connection with the electrode 1 and the workpiece 2. Typically, the wires are several meters long and the resulting inductance is significant, since it will tend to make the current across the gap constant and, thereby extend the period of current flow. As a result of this wiring inductance, a high voltage is generated which can break down switching element S.sub.2 if it is a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or other transistor, thereby adversely affecting the circuit reliability.
FIG. 27 is a circuit diagram showing an example of the prior art using transistors TR.sub.1 and TR.sub.2 as the switches S.sub.1, S.sub.2 of the foregoing circuit. The switching element TR.sub.2 is ON-OFF controlled by an oscillation control circuit 3. The wiring inductance L.sub.3 is not shown but remains effective in this circuit. FIG. 28 is a timing chart showing in waveform (a) the inter-electrode voltage, in waveform (b) the discharge current, in waveforms (c) and (d) the ON-OFF timing of the switching elements TR.sub.2 and TR.sub.1, and in waveform (e) the current flowing through the coil L.sub.1. The discharge current waveform (b) may be controlled by a waveform control method called "slope control". This method is particularly effective for low consumption of the electrodes. According to this method, the discharge current seen in waveform (b) is steeply raised to a base current level having a value I.sub.B and is then increased at a predetermined rate to, and maintained at the peak value I.sub.P, from which it then should drop rapidly. Specifically, with reference to the coil current in waveform (e), initially during period t.sub.1, switching element TR.sub.1 is switched between ON-OFF states (waveform d) to set the current of the coil L.sub.1 at I.sub.B. The current is set in a step-wise fashion, as seen in waveform (e). For a period t.sub.2, when TR.sub.1 is ON, the current flows at an increasing rate via a path including the auxiliary power source B.sub.2, the switching element TR.sub.1, the coil L.sub.1 and the diode D.sub.1. The increasing rate is expressed by the following Equation if the coil L.sub.1 has an inductance L (.mu.H): EQU di/dt=E.sub.2 /L (A/.mu.s) (1).
Then, for a period t.sub.3, while both TR.sub.2 and TR.sub.1 are OFF, the current flows at a decreasing rate via a path including the DC power source B.sub.1, the diode D.sub.2, the coil L.sub.1 and the diode D.sub.1. The decreasing rate is expressed by the following Equation: EQU di/dt=E.sub.1 /L (A/.mu.s) (2).
At time t.sub.4, the switching element TR.sub.2 is turned ON, while switching element TR.sub.1 continues to turn ON and OFF, to impress the so called "unloading voltage" (E.sub.1) between the electrode 1 and workpiece 2. Then, if a discharge takes place in the gap between the electrode and workpiece at time t.sub.5, the current flows at a generally increasing rate during a period between times t.sub.5 and t.sub.8 through a path including the DC power source B.sub.1, the auxiliary power source B.sub.2, the switching element TR.sub.1, the coil L.sub.1, the electrode 1, the workpiece 2 and the switching element TR.sub.2. On discharge the voltage across the gap will decrease from E.sub.1 to V.sub.arc, the arc voltage at which the arc discharge is extinguished. This This increasing rate is expressed by the following Equation: EQU di/dt=(E.sub.1 +E.sub.2 -V.sub.arc)/L (A/.mu.s) (3).
If during the period between times t.sub.5 -t.sub.8, when TR.sub.2 is ON, the switching element TR.sub.1 is turned ON for a period t.sub.6, the current flows at an increasing rate through a path including power supplies B.sub.1 and B.sub.2, switching elements TR.sub.1 and TR.sub.2, coil L.sub.1, electrode 1 and workpiece 2. This increasing rate is expressed by: EQU di/dt=(E.sub.1 +E.sub.2 -V.sub.arc)/L (A/.mu.s) (4).
However, when TR.sub.1 is turned OFF for a period t.sub.7, the current flows at a decreasing rate through a path including the diode D.sub.2, the coil L.sub.1, the electrode 1, the workpiece 2 and the switching element TR.sub.2. This decreasing rate is expressed by: EQU di/dt=V.sub.arc /L (A/.mu.s) (5).
If both the switching elements TR.sub.1 and TR.sub.2 are turned OFF at time t.sub.8, the discharge current is blocked, as seen in waveform (b). As a result, the current I.sub.L through the coil L.sub.1 (the "breaking current") will flow at a decreasing rate for a period t.sub.9, until it reaches level I.sub.B, as seen in waveform (e). During period t.sub.9, the breaking current I.sub.L flows along a path including the DC power source B.sub.1, the diode D.sub.2, the coil L.sub.1 and the diode D.sub.1. The decreasing rate is expressed by: EQU di/dt=E.sub.1 /L (A/.mu.s) (6).
Hence, the time period for the current I.sub.L to decrease from I.sub.P to I.sub.B, i.e. the dropping period of the breaking current, is calculated by: EQU T=L(I.sub.P -I.sub.B)/E.sub.1 (.mu.s) (7).
In the case of waveform control by the slope control method in which the current is increased at an increasing rate from the value I.sub.B, defined by I.sub.B .ltoreq.I.sub.P (I.sub.B .gtoreq.0) as illustrated in FIG. 28, it is necessary to allow I.sub.L to drop to reach the condition of I.sub.L =I.sub.B between cycles. As a result, the circuit of FIG. 27 is troubled by a problem that the quiescent period of the breaking current cannot be made shorter than the time period T of the foregoing Equation (7). In order to shorten the time period T, it is sufficient to insert a resistance into the path during the time period t.sub.9 to thereby change the energy to heat. However, this modification is contrary to the aforementioned goal of energy economy.